Sewage sludge treatment in a thermophilic membrane reactor (TMR): factors affecting foam formation

Comparison of methods for detecting protein extracted from excess activated sludge

The protein contents of hydrolyzed sludge supernatant are commonly determined with the Kjeldahl method, but this method suffers from complicated operations, long process times, and large quantities of chemicals consumed. In this paper, the Lowry, bicinchoninic acid (BCA), and Bradford methods were used to test the precision and spiked recovery of proteins from sludge supernatants hydrolyzed by alkaline-thermal hydrolysis (ATH), enzymatic hydrolysis (EH), and ultrasound-assisted enzymatic hydrolysis (UEH), and the results were compared with those obtained with the Kjeldahl method. For all the hydrolytic processes, the sludge protein values determined with the three tested methods were within 0.05 of each other, which met the experimental requirement for accuracy. Both the Lowry and BCA methods had recovery rates of 95-105%, while the Bradford method showed large deviations and was not highly reliable. The three protein determination methods showed significant differences with the Kjeldahl method (P<0.05). However, the relative deviation between the Kjeldahl and BCA methods was the smallest (3-5%), followed by those between the Kjeldahl and the Lowry (11-21%) and Bradford methods (21-90%), and the causes of the deviations were analyzed based on the protein hydrolysate components and the mechanisms for the different detection methods. On the basis of these results, the BCA method was chosen as the most appropriate quantification method for use with sludge protein extraction, and it was used to analyze the protein contents extracted from residual sludge samples obtained from two sewage treatment plants. The reliability of the method was verified, and this lays a foundation for the extraction and reclamation of sludge proteins.

Analysis on the properties of hydrolyzed amino acids in typical municipal sludge

In this study, amino acids, proteins, and microbial communities in sludge from different wastewater treatment plants (WWTPs) were analyzed. The results showed that the bacterial communities of different sludge samples were similar at the phylum level, and the dominant bacterial species in sludge samples with the same treatment process were the consistent. The main amino acids in EPS of different layers were different, and the amino acid results of different sludge samples were quite different, but the content of hydrophilic amino acids in all samples was higher than that of hydrophobic amino acids. And the total content of glycine, serine, and threonine related to sludge dewatering was positively correlated with protein content in sludge. In addition, the content of nitrifying bacteria and denitrifying bacteria in sludge was also positively correlated with the content of hydrophilic amino acids. In this study, the correlations between proteins, amino acids, and microbial communities in sludge were analyzed respectively, and the internal relationship was found. And it provided ideas for further study of sludge dewatering characteristics in the future.

Enhancing enzyme activity via low-intensity ultrasound for protein extraction from excess sludge

Rich protein within excess sludge could be recovered to prepare high value-added products such as liquid fertilizer and foaming agents. Low-intensity ultrasonication was adopted to help extract sludge protein by improving enzyme activity. Alkaline protease was added to the sludge for ultrasonic irradiation, and the maximum enzyme activity at 3500 kJ/kg TS was approximately 21% higher than that without ultrasonication. The protein extraction effect, specific resistance of sludge (SRS) and economics of low-intensity ultrasound-assisted enzymatic hydrolysis (LUEH) were compared with those of single enzymatic hydrolysis (EH) and HUEH under optimal conditions. The protein extraction rates of HUEH and LUEH were both higher than that of EH. Although the protein extraction rate of LUEH was 13.6% lower than that of HUEH, the amino acid content was similar because the low-intensity ultrasonic radiation promoted the enzyme activity and thereby enhanced the protein hydrolysis capacity. After hydrolysis, the SRS of LUEH was lower than that of HUEH, indicating that LUEH possessed a better dewatering performance, which was beneficial to the subsequent separation of the protein solution. The amount consumed by LUEH was approximately 20% lower than that consumed by HUEH and 17.3% lower than that consumed by EH. In addition, the enzyme dosage was reduced by approximately 38.5% with LUEH. Therefore, the total cost of LUEH was less than that of EH and HUEH, indicating that LUEH is more economically feasible for the extraction of protein from excess sludge.

Multi-scale analysis of the foaming mechanism in anaerobic digestion of food waste: From physicochemical parameter, microbial community to metabolite response

Foaming is a key issue that threatens the efficient and stable operation of the anaerobic digestion process. This study introduced three disturbances to induce foaming and explored the responses of physicochemical parameters, microbial communities, and metabolites to reveal the foaming mechanism. Under the three disturbance conditions, extracellular polymeric substances (EPS)-related parameters are significantly positively correlated with foam height, and EPS may cause foam by lowering the surface tension. Microorganisms that are more tolerant to high acid or high ammonia stress environments were identified after foaming, and they could resist the stress environment by producing more EPS. The up-regulated expression of sphingomyelin or ceramide was discovered after foaming, involved in the signal molecular transduction process of cell apoptosis or necrosis, which might be related to EPS production. Pantothenic acid involved in pantothenate and CoA biosynthesis pathways was down-regulated expression after foaming, which might be related to the hindered degradation of EPS. The response of multi-scale parameters in the foaming process shows that EPS is the key factor in foaming events.

Inhibition of Maillard reaction during alkaline thermal hydrolysis of sludge

The Maillard reaction (MR) occurs during the alkaline thermal hydrolysis (ATH) of sludge, which affects the quantity and quality of recovered protein. In this paper, four different sulfites were added to investigate their inhibitory effects on melanoidin production. The results showed that sulfites inhibited melanoidin production during ATH of sludge and the inhibitory rate increased with their concentration. At a concentration of 5.71 g/L, the inhibitory rates of NaHSO₃ on melanoidin were 63.27%. Furthermore, the 3D-EEM (Three-Dimension Excitation-Emission-Matrix) fluorescence spectroscopy and protein testing data showed that the inhibition of melanoidin production was accompanied by an increased protein concentration, and protein increased with increasing sulfites concentration. A 2.5-fold increase in protein concentration with Na₂S₂O₄ significantly enhanced the quantity of protein recovered. Therefore, the addition of sulfite during ATH of sludge reduces the amount of non-biodegradable melanoidin, which in turn benefits protein recovery.

Effect of extracellular polymeric compositions on in-situ sludge minimization performance of upgraded activated sludge treatment for industrial wastewater

The sludge yield minimization from advanced biological treatment for industrial wastewater could be considered a poorly explored area, therefore, seeks serious attention of the scientific community. Up to best of the knowledge, the extracellular polymeric substances (EPS) profile underlying an upgraded activated sludge treatment (as MANODOX system) for real tannery wastewater has not been addressed in a desired manner. This study covers the elucidation of EPS degradation mechanism and floc morphology underlying MANODOX system for the treatment of real tannery influent. For this purpose, a modified heat extraction method was followed for the estimation of EPS fractions like protein (PN), polysaccharides (PS) and humic contents from the sludge. For the present investigation, the variation in floc characteristics including PN/PS ratio, sludge hydrophobicity, sludge volume index, and facultative microbiota at corresponding change in hydrodynamic sludge retention time (SRT) of 08-40 days was emphasized. The strict maintenance of adapted operational strategies including favoring range of SRT (24 days) for MANODOX implementation succeeded an outstanding in-situ sludge yield minimization lowered up to 0.39 gMLSS/gTCOD that attributed to three times lowered accumulation of PN and PS, comparably lower PN/PS ratio, higher salinity of the mixed liquid, weakened cell-to-cell attachment compared with a parallel run identical aerobic treatment. Here, the reason for improved hydrophobicity and corresponding decline in floc aggregation was attributed to change in sludge PN/PS ratio, carbon to nitrogen ratio of feed influent. The observations confirmed that the sludge yield minimization from MANODOX like systems could be effectively controlled by maintaining aforementioned operational tactics.

The removal of ammonia nitrogen via heterotrophic assimilation by a novel Paracoccus sp. FDN-02 under anoxic condition

A novel strain FDN-02 was isolated from a sequencing batch biofilm reactor. FDN-02 was identified as Paracoccus sp., and the Genbank Sequence_ID was MW652628. Comparing with the removal efficiency of ammonia nitrogen (NH₄⁺-N) by bacterium FDN-02 under different growth conditions, the optimal initial pH, carbon source, and C/N ratio were 7.0, sucrose, and 16, respectively. The maximum removal efficiency and rate of NH₄⁺-N were respectively 96.2% and 10.06 mg-N/L/h within 8 h under anoxic condition when the concentration of NH₄⁺-N was 44.87 mg/L. Specifically, 71.9% of NH₄⁺-N was utilized by strain FDN-02 through heterotrophic assimilation to synthetize organic nitrogen, and approximately 24.1% of NH₄⁺-N was lost in the form of gaseous nitrogen without the emission of nitrous oxide. Bacterium FDN-02 was also found to be a denitrifying organism, and nitrate nitrogen and nitrite nitrogen of lower concentrations were removed by denitrification after the enlargement of biomass. Further investigation showed that the biomass after the removal of NH₄⁺-N by strain FDN-02 had resource utilization potential, and the contents of proteins and amino acids were 635 and 192.97 mg/g, respectively, especially for the usage as an alternative nutrient source for livestock and organic fertilizers. This study provided a promising environmentally friendly biological treatment method for the removal of NH₄⁺-N in the wastewater.

Stimulation of Lipid Extraction Efficiency from Sewage Sludge for Biodiesel Production through Hydrothermal Pretreatment

In this study, two types of sewage sludge (primary sludge and waste activated sludge) were hydrothermally treated at 125–250 °C to enhance the lipid extraction efficiency and obtain a higher biodiesel yield. The enhanced efficiency of the lipid extraction method was compared with the efficiency of the organic solvent extraction method. The results confirmed that a hydrothermal reaction could be an appropriate option for disrupting sludge cell walls and increasing the lipid extraction from sewage sludge. The highest lipid recovery efficiency was observed at 200 °C, and the lipid recovery efficiency of primary sludge and waste activated sludge increased from 7.56% and 5.35% to 14.01% and 11.55% by weight, respectively. Furthermore, transesterified lipids, such as biodiesel from sewage sludge, mostly consist of C16 and C18 methyl esters, and have features similar to those of jatropha oil-based biodiesel. During the hydrothermal treatment, the carbon content in the sludge decreased as the carbon transformed into lipids and the lipids were extracted. The volatile matter and fixed carbon content in the solid residue decreased and increased, respectively, through chemical dehydration and decarboxylation reactions under hydrothermal reaction conditions.

Evaluation of foaming potential for water treatment: limits and developments

The critical issue generated by foaming in wastewater treatment plants (WWTPs) is a problem that is currently very common and shared, but which to date is treated mainly only at the management level. In this work, an experimental study with foam tests on real and synthetic waters was conducted using a laboratory scale plant and foaming power indices were calculated. To date, the estimation of foaming potential is mainly based on these indices which give information only on height/volume of foams but not on the type of foams, in terms of consistency and therefore stability. Tests showed that foaming power indices were highly variable with the same water: it was not possible to identify a single foaming potential value for each water. Two models were proposed to estimate the percentage increase in height of chemical foams produced following the introduction of air below the surface of a liquid. In terms of determination coefficient, the results obtained from the complex model were better: R² was 0.82 for the simple linear model and 0.90 for the complex one. This approach has allowed to underline some critical aspects of foaming potential as it is determined today and the possible improvements applicable for a more objective evaluation.

Comparison of protein extraction methods from excess activated sludge

In this study, we used chemical methods (acid-thermal (AT) and alkaline-thermal (AKT)), enzymatic methods (single enzyme (SE) and composite enzyme (CE)) and assisted enzymatic methods (ultrasonic assisted enzyme (USE) and thermal assisted enzyme (TE)) to extract proteins from excess activated sludge. The advantages and applicability of each method were compared and analyzed in terms of their protein extraction rate (R_P), energy consumption, material consumption and cost, protein hydrolysates and sludge dewatering performance. The results showed that the R_P of the chemical methods were more than 75%, which were much higher than those of the enzymatic methods. Moreover, the R_P of SE was significantly strengthened by physical means (ultrasonic or thermal), and the average R_P was increased by more than 39% compared with that of enzymatic methods. The energy consumption analysis showed that chemical methods consumed significantly more energy than the enzymatic methods. Further analysis of enzymatic methods and assisted enzymatic methods revealed that although the energy consumption of USE was similar to that of SE, its enzyme consumption and cost were lower. In addition, the proteins extracted by USE had a high content of amino acids, which was suitable for the preparation of animal feed. The proteins extracted by AKT had a high content of polypeptides, which was beneficial to the preparation of a protein foaming agent. Furthermore, the sludge dewatering performance after hydrolyzation by the six hydrolysis methods was significantly improved, which was beneficial to the separation of proteins.

Valorization of agro-industry residues in the building and environmental sector: A review

Environmental pollution has become a relevant issue as the population rises and resources decrease. Reuse and recycling still have the greatest potential as they turn the waste into a new resource, representing the 'closed-loop' step of a circular economy (CE). Looking for new applications for agro-industry waste represents both an environmental issue, as its incorrect disposal is a cause of pollution, and a chance to exploit zero-cost natural wastes. The present review, with around 200 articles examined, focuses on possible reuses of these residues in (a) building construction, as additives to produce thermal and acoustic insulation panels, and (b) in water treatments, exploited for removal of pollutants. The selected materials (coconut, coffee, corn, cotton and rice) have industry production wastes with suitable applications in both sectors and huge worldwide availability; their reuse may thus represent a new resource, with an impact based on the production rate and the possible replacement of current inorganic materials. Along with possible implementation of the selected materials in the building industry and environmental engineering, a brief description of the production and supply chain are provided.

Protein extraction and sludge dewatering performance of ultrasound-assisted enzymatic hydrolysis of excess sludge

Excess sludge contains a high amount of protein, which can be recovered to prepare protein foaming agents and other products with high added value. Enzymatic hydrolysis (EH) is a promising technology for the recovery of protein from excess sludge, and ultrasound has been identified as a potential method to assist in sludge disintegration. Ultrasonic pretreatment was combined with alkaline protease hydrolysis to extract protein from excess sludge produced by A²/O treatment (S1) and an oxidation ditch treatment (S2), and the extraction effects and changes in sludge dewatering performance were studied. The effects of the six factors ultrasonic power density, ultrasonication time, enzyme dose, pH, hydrolysis temperature and hydrolysis time were analyzed. The results showed that the ultrasound-enhanced enzymatic method could effectively extract sludge protein. Although the extraction efficiencies for the different municipal sludges were different, their extraction conditions were relatively similar. Considering the protein extraction rate and sludge dewatering performance, the selected extraction conditions were as follows: ultrasonic power density, 1 W/mL; ultrasonication time, 20 min; enzyme dose, 3500 U/g; pH 11; hydrolysis temperature, 60 °C; and hydrolysis time, 3 h. Under these conditions, the protein extraction rate (R_p) of S1 and S2 reached 55.9% and 52.3%, respectively. Moreover, the improvement in sludge dewatering performance (D_w) of S1 and S2 was 49.5% and 52.4%, respectively. Comparison of the protein, polypeptide, and amino acid contents obtained from ultrasound-assisted enzymatic hydrolysis (UEH), EH, and ultrasonic hydrolysis (UH) further demonstrated the beneficial effect of ultrasound application on enzymatic hydrolysis.

Foams in Wastewater Treatment Plants: From Causes to Control Methods

The formation of persistent foams can be a critical problem in wastewater treatment plants (WWTPs) as it could lead to a series of operational problems, especially the reduction of the overall system performance. To date, the effects of foaming in the WWTPs are a problem that is currently very common and shared, but which to date is treated mainly only at the management level and still too little studied through a globally shared scientific method: the complexity of the phenomenon and the systems have led to numerous partially contradictory descriptions and hypotheses over the years. The goal must be to suggest future research directions and indicate promising strategies to prevent or control the formation of foams in WWTPs. This study examines and investigates the problem of foams by a methodological approach of research through a review on the state of the art: the factors influencing the formation of foams are described first (such as surfactants and/or extracellular polymeric substances (EPSs)), then the known methods for the evaluation of foaming, both direct and indirect, are presented, with the aim of identifying the correct and best (from the management point of view) control and/or prevention strategies to be applied in the future in WWTPs.

Peracetic acid for conditioning of municipal wastewater sludge: Hygienization, odor control, and fertilizing properties

Peracetic acid (PAA) is an environmentally friendly disinfectant and oxidizer used in several water and wastewater treatment applications. In the present study, PAA was utilized for the conditioning of municipal wastewater sludge before thickening and dewatering. It was shown that PAA can effectively prevent odor formation (i.e., H₂S and NH₃) and provide hygienization (using E. coli and Salmonella as indicators). Phytotoxicity can be prevented by controlling the amount PAA-conditioned sludge that is mixed in the soil to be fertilized. The required PAA dose for hygienization was relatively high (480 mg 100% PAA perL sludge) but the results indicated that other sludge stabilization processes are not necessarily required. Therefore, the proposed process involving PAA could be feasible in cases where limited land area is available for sludge processing or quick conditioning of sludge is required.

The reuse of biosolids on agricultural land: Critical issues and perspective

The sludge management represents a considerable amount of operational costs for wastewater treatment plants (WWTPs). Issues concerning the treatment and the recovery/disposal of biosolids are gaining importance especially in Lombardy region, where the biosolid landspreading on agricultural soils is a very common practice. The aim of this work was to evaluate the results obtained from a survey carried out on the biosolids/sludge, derived from WWTPs, ingoing to and outgoing from the STPs (sludge treatment plants-authorized for the treatment on behalf of third parties) located in Pavia Province. Moreover, the characterization of agricultural soils that receive the biosolids is carried out. Furthermore, the whole biosolid management chain, from production to landspreading on agricultural soils, was studied, highlighting the critical issues based on the survey results. The results obtained suggested the following actions: (a) the reduction of sludge production in WWTPs; (b) a more "controlled" production in terms of biosolid qualitative characteristics; (c) better "selection" of the sludge ingoing to STPs; and (d) more effective actions to control the "response" of the agricultural soils. Furthermore, full compliance with the best spreading practices on the soils is required, as well as a better use of agronomic skills to obtain a higher resource valorization. PRACTITIONER POINTS: In this work, the characteristics of biosolid spread in agricultural soils in Pavia Province were analyzed. The minimization of sludge production in WWTPs is encouraged in order to reduce critical issues related to biosolid management. The stabilization process in WWTPs should improve in order to obtain a better quality sludge. A better "selection" of the sludge ingoing to STPs and a more effective monitoring of the agricultural soils are encouraged.

Treatment of aqueous wastes by means of Thermophilic Aerobic Membrane Reactor (TAMR) and nanofiltration (NF): process auditing of a full-scale plant

This work focuses on the Thermophilic Aerobic Membrane Reactor (TAMR) process. The research was carried out on a full-scale facility where, all along a 12-year period, daily monitoring and process audit tests were conducted for the process analysis and optimization. The plant treated -light and high-strength aqueous wastes and two different configurations were adopted: (1) thermophilic biological reactor + ultrafiltration (TAMR) and (2) TAMR + nanofiltration (TAMR + NF). In the latter case, the average chemical oxygen demand removal yield was equal to 89% and an effective denitrification (nitrogen oxides removal equal to 96%) was achieved by reducing the dissolved oxygen concentration in the bioreactor. Low specific sludge production was observed. Poor sludge settling properties were measured by a lab-scale settling test; respirometric tests (nitrogen uptake rate and ammonia uptake rate) showed the presence of denitrification and the inhibition of nitrification. Hydrodynamic tests revealed the presence of a significant dead space, thus showing room for improving the overall process performance. Finally, the rheological properties of the sludge were measured as a function of the biomass concentration, pH, temperature, and aeration scheme.

Legislation for the Reuse of Biosolids on Agricultural Land in Europe: Overview

The issues concerning the management of sewage sludge produced in wastewater treatment plants are becoming more important in Europe due to: (i) the modification of sludge quality (biological and chemical sludge are often mixed with negative impacts on sludge management, especially for land application); (ii) the evolution of legislation (landfill disposal is banned in many European countries); and (iii) the technologies for energy and material recovery from sludge not being fully applied in all European Member States. Furthermore, Directive 2018/851/EC introduced the waste hierarchy that involved a new strategy with the prevention in waste production and the minimization of landfill disposal. In this context, biological sewage sludge can be treated in order to produce more stabilized residues: the biosolids. In some European countries, the reuse of biosolids as soil improver/fertilizer in arable crops represents the most used option. In order to control the quality of biosolids used for land application, every Member State has issued a national regulation based on the European directive. The aim of this work is to compare the different approaches provided by European Member States for the reuse of biosolids in agricultural soils. A focus on the regulation of countries that reuse significant amount of biosolids for land application was performed. Finally, a detailed study on Italian legislation both at national and regional levels is reported.

Recent Progresses in Application of Membrane Bioreactors in Production of Biohydrogen

Biohydrogen is a clean and viable energy carrier generated through various green and renewable energy sources such as biomass. This review focused on the application of membrane bioreactors (MBRs), emphasizing the combination of these devices with biological processes, for bio-derived hydrogen production. Direct biophotolysis, indirect biophotolysis, photo-fermentation, dark fermentation, and conventional techniques are discussed as the common methods of biohydrogen production. The anaerobic process membrane bioreactors (AnMBRs) technology is presented and discussed as a preferable choice for producing biohydrogen due to its low cost and the ability of overcoming problems posed by carbon emissions. General features of AnMBRs and operational parameters are comprehensively overviewed. Although MBRs are being used as a well-established and mature technology with many full-scale plants around the world, membrane fouling still remains a serious obstacle and a future challenge. Therefore, this review highlights the main benefits and drawbacks of MBRs application, also discussing the comparison between organic and inorganic membranes utilization to determine which may constitute the best solution for providing pure hydrogen. Nevertheless, research is still needed to overcome remaining barriers to practical applications such as low yields and production rates, and to identify biohydrogen as one of the most appealing renewable energies in the future.

What Advanced Treatments Can Be Used to Minimize the Production of Sewage Sludge in WWTPs?

Similar to other types of waste, sewage sludge (SS) must be minimized, not only to respect the European Directive 2018/851 on waste, but also because the cost of sludge management is approximately 50% of the total running costs of a wastewater treatment plant (WWTP). Usually, minimization technologies can involve sewage sludge production with three different strategies: (i) adopting a process in the water line that reduces the production of sludge; (ii) reducing the water content (dewatering processes) or (iii) reducing the fraction of volatile solids (stabilization). This review, based on more than 130 papers, aims to provide essential information on the process, such as the advantages, the drawbacks and the results of their application. Moreover, significant information on the technologies still under development is provided. Finally, this review reports a discussion on the impact of the application of the proposed processes in the sludge line on a WWTP with a capacity exceeding 100,000 population equivalent (PE).

Wastewater treatment by means of thermophilic aerobic membrane reactors: respirometric tests and numerical models for the determination of stoichiometric/kinetic parameters

Existing wastewater/aqueous waste treatment plants often need to be upgraded in order to improve their performance. The satisfactory operation of biological treatment plants requires appropriate monitoring, and respirometric techniques are needed to determine the kinetic parameters that regulate biological processes. Innovative technologies are treating industrial wastewater/aqueous waste, such as thermophilic aerobic treatments. Thermophilic aerobic biological systems operate at temperatures higher than 45°C. Such temperature levels can be reached, at a reasonable cost, using wastewater with a high organic loading and reactors, which are appropriately thermally insulated. This kind of treatment shows high removal kinetics of biodegradable substrates and a very low sludge production. This paper describes the application of respirometric tests in thermophilic conditions on the biomass derived from a thermophilic aerobic membrane reactor in order to model the process, with a particular focus on the rapidly biodegradable chemical oxygen demand (rbCOD). The utility of rbCOD determination is related to the optimal treatment that the aqueous waste should undergo. Calculating the kinetic parameters is critical to the biological modelling used in the management and control of wastewater treatment plants.

Treatment of high strength aqueous wastes in a thermophilic aerobic membrane reactor (TAMR): performance and resilience

In the present work, the thermophilic aerobic membrane reactor technology was studied for the treatment of high strength aqueous wastes mainly containing dyes, surfactants and solvents. The thermophilic biomass resilience and the process stability under critical conditions (such as rapid rise of the mixed liquor pH, oxygen supply interruption, etc.) were also evaluated. The experimental work was carried out with the use of a pilot plant at semi-industrial scale, which was managed throughout for 14 months; the operation temperature was 49 °C and the organic loading rate was increased from 3 to 12 kg_COD m^-3 d^-1. Critical conditions, especially the interruption of oxygen supply, affected the pilot plant performance but did not cause a complete system break down. After the temporary reduction of process performance, also proven by the decrease in the oxygen consumption, the normal working conditions were restored. Moreover, the longer non-aerated phase involved a significant reduction (40%) of volatile suspended solids concentration in the biological reactor and the increase of 30% in foaming power; nevertheless, once the oxygen supply was reactivated, optimal conditions were rapidly restored. Therefore, the study showed the high resilience of the thermophilic biomass, which was able to recover full functionality after critical events.

Thermophilic membrane bioreactors: A review

This study undertakes a state-of-the-art review on thermophilic membrane bioreactors (ThMBRs). Thermophilic aerobic membrane bioreactors (ThAeMBR) and thermophilic anaerobic membrane bioreactors (ThAnMBR) have been widely tested for various high-temperature industrial wastewater treatments at lab- and pilot-scale studies and full-scale applications. The biological and membrane performances of the ThAeMBRs and ThAnMBRs could be better, comparable or poorer, as compared to the mesophilic ones. In general, sludge yield was much lower, biodegradation kinetic was higher, and microbial community was less diversity in the ThAeMBR and ThAnMBR systems. The results from the literature show that ThMBR technology has demonstrated many advantages and is a promising technology for industrial wastewater treatment and sludge digestion. Furthermore, challenges and opportunities of various ThMBRs for industrial applications are identified and discussed.

Investigation of foaming causes in three mesophilic food waste digesters: reactor performance and microbial analysis

Foaming negatively affects anaerobic digestion of food waste (FW). To identify the causes of foaming, reactor performance and microbial community dynamics were investigated in three mesophilic digesters treating FW. The digesters were operated under different modes, and foaming was induced with several methods. Proliferation of specific bacteria and accumulation of surface active materials may be the main causes of foaming. Volatile fatty acids (VFAs) and total ammonia nitrogen (TAN) accumulated in these reactors before foaming, which may have contributed to foam formation by decreasing the surface tension of sludge and increasing foam stability. The relative abundance of acid-producing bacteria (Petrimonas, Fastidiosipila, etc.) and ammonia producers (Proteiniphilum, Gelria, Aminobacterium, etc.) significantly increased after foaming, which explained the rapid accumulation of VFAs and NH4+ after foaming. In addition, the proportions of microbial genera known to contribute to foam formation and stabilization significantly increased in foaming samples, including bacteria containing mycolic acid in cell walls (Actinomyces, Corynebacterium, etc.) and those capable of producing biosurfactants (Corynebacterium, Lactobacillus, 060F05-B-SD-P93, etc.). These findings improve the understanding of foaming mechanisms in FW digesters and provide a theoretical basis for further research on effective suppression and early warning of foaming.